This invention relates to an internal combustion engine and more particularly to a variable valve drive system for such engines.
In order to improve the performance of an engine through a wide range of engine running conditions (speed and load), it has been proposed to utilize a system wherein the degree of lift or opening of one or more valves of the engine can be controlled. A number of different types of variable valve lift mechanisms have been employed but, for the most part, the range of valve lift adjustment that is possible is quite limited. This is basically because of the geometry of the systems, which have been proposed, and the operational methods, which they have employed.
For example, in one form of arrangement, a rocker arm is interposed between the cam lobe and the valve or valve actuating member. This rocker arm has its pivotal axis adjustable so as to in affect change the lever ratio and thus, alter the valve lift. An example of this type of valve lift control is shown in published Japanese Application Hei 7-91217 (U.S. Pat. No. 5,501,186).
As may be seen in this prior art construction, the rocker arm interposed between the cam lobe and the valve tappet has its pivot axis shifted in a transverse direction so as to change the effective lift of the valve. This is because the lever arm ratio is in effect changed. However, there are several disadvantages of this type of construction. First, the shifting of the pivot axis also causes the point of contact between the rocker arm and valve actuator to shift. Thus, the problem of wear in this area is aggravated and uniform wear is also made more difficult. Furthermore, since the shifting must be generally confined to the area immediately adjacent the axis of the actuated valve, the amount of variation in lift possible is restricted.
Another embodiment is shown in that application wherein different pivot points are utilized by having different pivot arrangements for the valve actuating rocker arm. However, this further limits the degree of adjustment and also causes the problem of having only step wise adjustment rather than adjustment over a range of positions.
Another type of system for adjusting the valve lift of an engine during running is shown in published Japanese Application Hei 6-307219. In this type of arrangement, the lever ratio of the rocker arm is in effect changed by providing a rolling follower that is interposed between the cam lobe and an upper surface of the rocker arm. This roller follower is shifted transversely to change the effective lever ratio. Again, however, this severely limits the degree of adjustment that can be made and also further complicates the overall structure. Furthermore, this system does not permit zero or not nearly zero lift for the valve actuation. In other words it is not possible to disable the valve operation.
It is, therefore, a principal object to this invention to provide an improved valve actuating system for an internal combustion engine wherein large degrees of variation in valve lift is possible.
It is a further object to this invention to provide a relatively simple and yet continuously variable valve lift control mechanism for the valves of an internal combustion engine.
It is a yet further object to this invention to provide an arrangement for controlling the valve lift an internal combustion engine that is compact in nature, while at the same time, maintains a large effective variation in valve lift.
This invention is adapted to be embodied in a variable valve operating system for the valves of an internal combustion engine and specifically one, which is capable of varying the degree of opening in response to the rotation of the cams to a large extent. The valve operating device comprises a rocker arm having a follower portion engaged by the cam. A pivot axis about which the rocker arm pivots in response to the action of the cam on the follower portion is provided. A valve operating portion is operatively engaged with the associated valve for effecting lift of the valve in response to pivotal movement of the rocker arm. A variable lift actuator is effective to change the distance between the point on the valve operating portion that operatively engages the valve without changing the point that is operatively engaged and the distance between that point and the pivot axis of the rocker arm so as to change the degree of lift between substantially no lift to substantially maximum amount of lift.